Localisation of deposited filling gases (CO_2) in cryogenic thermal insulation materials using neutron radiography

摘要

The local distribution of deposited filling gases (CO_2) within a sample consisting of a packed bed of ~1 ìm-sized solid glass spheres exposed to a temperature gradient at cryogenic temperatures was investigated by neutron radiography measurements. This local distribution is useful for the understanding of the degradation of the thermal conductivity in cryogenic thermal insulations and the technique of deposition-evacuation (cryoevacuation). As neutron radiography is expected to be sensitive to the redistribution, a special sample cell was designed which allows establishing a typical cryogenic temperature gradient (77-293 K). The two-dimensionally resolved transmission of the neutron flux by the cell reveals that all CO_2 is deposited preferentially at and close to the cold wall. The form of the spatial CCO_2-distribution is not affected by the total amount of CO_2 injected into the sample.